Meredith effect¶
Aerodynamic recovery of some cooling-system drag when waste heat added to a shaped duct's airflow increases downstream momentum, subject to pressure and installation losses.
Core Idea¶
The Meredith effect is partial recovery of an aircraft cooling installation's aerodynamic drag when rejected heat enters moving air in a shaped duct and contributes to downstream momentum. The heat exchanger and duct also create losses, so recovery does not promise net thrust. F. W. Meredith analyzed this possibility for enclosed aircraft radiators.[ref-818d4545d8a4][ref-5fc536c1a6d1]
Scope of Application¶
The literal setting is a ducted aircraft cooling flow with a heat exchanger, rejected heat, and an aerodynamic outlet balance. Silverstein's 1939 tests used an electrical resistance unit to simulate radiator heating in expanded wing ducts; they tested the mechanism but were not a flying engine-waste-heat installation. A 2024 fuel-cell-aircraft design study used computer simulation, not flight testing.[ref-dca8ee7e65bc][ref-5fc536c1a6d1]
Clarity¶
Heat can improve a cooling duct's momentum balance while the full installation still has drag. “Less drag when heated” and “net propulsive thrust” are different claims. The type of evidence also matters: a theoretical design, a heated model experiment, and a computational aircraft design do not establish the same performance claim.[ref-818d4545d8a4][ref-dca8ee7e65bc][^ref-5fc536c1a6d1]
Manages Complexity¶
Identify the moving ducted air, rejected heat entering it, exchanger pressure loss, and downstream momentum. Compare the benefit from heat addition with resistance and installation drag at a stated condition. This map prevents one favorable component from standing in for the full cooling-system balance.[^ref-5fc536c1a6d1]
Abstract Reasoning¶
First establish that rejected system heat enters a shaped cooling-airflow path. Then compare the same duct with and without heat transfer at the stated flight condition, accounting for pressure and drag losses. A model heater can test heat-addition behavior, and a simulation can compare designs, but each requires its own evidence before flight performance is claimed.[ref-dca8ee7e65bc][ref-5fc536c1a6d1]
Knowledge Transfer¶
The same role map applies within aircraft cooling, from radiator-duct studies to proposed electric-aircraft heat-exchanger ducts. It does not turn every use of waste heat into a Meredith effect. The live Heat Engine entry describes a cyclic heat-to-work system; this one-pass cooling-duct recovery need not be such a system.
Example¶
Adler, Lamkin, and Martins simulated a fixed optimized duct at a modeled high-speed cruise point. Computed drag was 265 N without heat transfer and 77 N with heat transfer, about 71% less. Mapped back: the optimized duct carries cooling airflow; its modeled heat exchanger adds rejected heat; the drag comparison tests pressure loss and downstream momentum; the cruise design point supplies operating conditions. This is a within-duct computational comparison, not a flight result or universal thrust claim.[^ref-5fc536c1a6d1]
Neighborhood in Abstraction Space¶
Meredith effect sits in a sparse region of the domain-specific corpus (99th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.
Family — Unclustered & Miscellaneous (2551 abstractions)
Nearest neighbors
- Isothermal Process — 0.77
- Critical Heat Flux — 0.75
- Cooling — 0.75
- NTU method — 0.75
- Adiabatic Process — 0.75
Computed from structural-signature embeddings · 2026-10-08
Not to Be Confused With¶
An exposed radiator without shaped flow and momentum recovery does not instantiate this effect. A fuel-heated ramjet uses a different heat source. Silverstein's tests found partial recovery but judged it not of practical importance for the tested ducts up to 400 mph. The current catalog treats Meredith effect as a provisional unparented root after typed comparison with Heat Engine, Dissipation, and other related entries.[^ref-dca8ee7e65bc]
[^ref-818d4545d8a4]: F. W. Meredith, “Cooling of Aircraft Engines with Special Reference to Ethylene Glycol Radiators Enclosed in Ducts,” Aeronautical Research Committee Reports and Memoranda No. 1683 (1935), original report and abstract. https://reports.aerade.cranfield.ac.uk/handle/1826.2/1425 [^ref-dca8ee7e65bc]: Abe Silverstein, “Experiments on the Recovery of Waste Heat in Cooling Ducts,” NACA Special Report 111 (May 1939), original report abstract and introduction; the introduction describes the electrical resistance unit used to simulate a radiator. https://ntrs.nasa.gov/citations/20090015245 [^ref-5fc536c1a6d1]: Eytan J. Adler, Andrew H. R. Lamkin, and Joaquim R. R. A. Martins, “Ducted Heat Exchanger Thermal and Aerodynamic Shape Optimization,” ICAS 2024, paper 0080 (2024), especially §3.1 and Fig. 18, printed pp. 16–17, and p. 19 conclusions; Fig. 19 compares separately optimized designs. https://www.icas.org/icas_archive/icas2024/data/papers/icas2024_0080_paper.pdf